The present invention relates to a lithium secondary battery which is superior in safety, and has high weight energy density (energy stored per unit weight, hereinafter called “energy density”), and which is suitably used for, particularly, an electric vehicle.
In recent years, the lithium secondary battery is being rapidly and widely used to realize a small power source for portable electronic equipment. In addition, effort of development is being also made to realize practical use of the lithium secondary battery as a motor driving battery for an electric vehicle which replaces a gasoline-powered vehicle, and as a battery for storing electric power in the night.
The structure of lithium secondary battery is roughly divided into a wound type shown in FIG. 2 and a laminated type shown in FIG. 3. An internal electrode body 1 of the wound type is constituted by winding a positive electrode 2 and a negative electrode 3 through a separator 4, in which the positive electrode 2 with a large area or the like can be contained in a tubular container. In the case of this wound type, since it is sufficient that there is at least one lead 5 from each electrode 2, 3, and, even if it is desired to lower current collection resistance of each electrode 2, 3, it is sufficient to increase the number of leads, there is an advantage that the internal structure of the battery does not become complicated to make for easy assembly of the battery.
On the other hand, an internal electrode body 7 of the laminated type is constructed by alternately laminating positive electrodes 8 and negative electrodes 9 in multiple layers through separators 10, in which area per one positive electrode 8 or the like is not large, but the electrode area of the entire battery can be increased by laminating them in multiple layers. The internal electrode body 7 being produced can be designed into any desired shape including a rectangular parallelepiped, cylindrical or tubular shape depending on the shape of each electrode 8, 9 and the number of laminations. However, since a lead 6 is necessary for each electrode 8, 9, there is a disadvantage that the internal structure of the battery becomes complicated, and it is inferior to the wound type in view of assembly workability of the battery.
In both the wound and laminated type structures, the internal electrode body is housed in a metal battery case so that each electrode and lead do not contact each other. Conventionally, stainless steel is most widely used for this battery case, and sometimes nickel, titanium or the like may be used.
However, since stainless steel and nickel have higher specific gravities, there is a disadvantage that, when used for the battery case, the battery itself becomes heavy, so that the energy density is low. On the other hand, while titanium has an advantage to have lower specific gravity than stainless steel or nickel, and to be excellent in corrosion resistance, it is expensive, and its use is limited to a specific application such as space development, so that it is difficult to be used as a general purpose battery component. In addition, in the lithium secondary battery, the battery case itself is often used as a current path for the positive or negative electrodes, and such material has high electric resistance, leading to a cause of power loss. In addition, such metal is not always said to have good workability as the battery case.
Under such circumstances, a lithium secondary battery for an electric vehicle (EV) or hybrid electric vehicle (HEV) is required to have a cell capacity of at least 50 Wh, to be light enough so as not to increase the weight of the vehicle itself, and to have high safety. To meet such requirements, stainless steel having a high melting point and high strength has been conventionally used by particularly taking safety into consideration. However, as described earlier, it is difficult to solve the problem for reducing weight of the battery. In addition, EV and HEV require a high current in acceleration, and, when the battery case is used as the current path, magnitude of electric resistance of the battery case cannot be ignored, and there remains a problem on workability of the battery case when the size is increased. Also, when nickel or titanium is used, such problems are also difficult to be solved because of physical characteristics of these materials.
Then, to solve such problems, the inventors have studied the possibility to use aluminum as the battery case which has light weight, is excellent in electron conductivity, and of good workability. There is no precedent to use aluminum as a battery case for a large battery of 50 Wh or more. This may be because the melting point of aluminum is as relatively low as 660° C., a temperature significantly lower than those of the above materials, and, when the battery case is softened or melted due to erroneous use or the like, it is feared that the electrolyte will evaporate or burn, or explode in the worst case.
According to Battery Association of Japan, as the “Guideline for Safety Evaluation on Secondary Lithium Cells” (commonly called “SBA Guideline”), it regulates that even if the entire energy fully charged is instantaneously discharged by an external short-circuit or an internal short-circuiting caused by a nail piercing test or the like, and then the lithium secondary battery generates heat, the battery does not burst or burn.
While such safety is strictly required, the inventors found that, even when an aluminum battery case is used, the problems on safety could be solved by accurately measuring temperature rise on the surface of the battery to calculate specific heat of the battery, and identifying the relationship between battery capacity and weight, and that reduction of energy density could be prevented by optimizing the battery case shape, and thus reached the present invention.